Many structures have pitched, shingled roofs, which prevent water, e.g., rain water, from entering the structures by causing water to pass over the shingles and shed off the roofs. A pitched, shingled roof has a pitched substrate, such as a plurality of plywood sheets, with a plurality of shingles attached thereto.
Each shingle has an upper portion (i.e., a headlap portion) and a lower portion (i.e., an exposure portion) wherein the exposure portion is exposed to the environment. The shingles are typically attached to the substrate in rows known as courses wherein the exposure portion of an upper course of shingles overlaps the headlap portion of an adjacent lower course of shingles. For example, a first course of shingles may be attached to the substrate nearest the lowest point of the roof, i.e., the eave portion of the roof. A second course of shingles may then be attached to the substrate slightly higher on the roof than the first course. The shingles are placed so that the exposure portion of the second course of shingles overlaps the headlap portion of the first course of shingles. This overlapping continues with successive rows of shingles to the highest point on the area of the roof, i.e., the hip or the ridge.
To prevent alignment of the seams between shingles in adjacent courses (and thereby allow for a leak path through the shingles), the first shingle in each course may be cut shorter to create an offset shingle. Offset shingles are applied at the start of a course of shingles, and the width of the offset shingles in each course is varied so the seams between shingles in adjacent courses are not aligned.
Attaching the shingles to the roof is typically achieved by the use of nails or other fastening devices that pass through the shingles and into or through the substrate. The fastening devices are typically placed through the headlap portion of the shingles so that they are overlapped by shingles in an adjacent higher course as described above. This placement of the fasteners prevents water from entering the structure through holes caused by the fasteners.
Some roofs have a membrane (i.e., an underlayment) located between the substrate and the shingles. The membrane may, as an example, be conventional tar paper or other underlayment material that is nailed to the substrate. Strips of the membrane are typically attached to the roof in an overlapping fashion wherein an upper strip overlaps its adjacent lower strip. Accordingly, the membrane serves to shield the substrate from water should a shingle become damaged. For example, if a shingle becomes cracked or otherwise leaks, water will contact the membrane rather than the substrate. Water will then pass along the membrane without contacting the substrate or entering the structure.